In February, 1984, the DTM converted from manual to automated platelet collection techniques, acquiring for this four automated cell separation devices. During the next lO years, 26 donors undergoing apheresis procedures on the Fenwal CS-3000 device experienced acute hypersensitivity reactions. Sixteen reactions occurred during plateletpheresis and 10 reactions occurred during granulocytapheresis procedures. The purpose of this study was to identify the allergen responsible for these reactions. Using a combination of skin tests, radioallergosorbent tests, and basophil histamine release assays, specific IgE-mediated sensitization ethylene oxide, a gas used to sterilize the plastic disposable apheresis kits, was found in 10 of 16 plateletpheresis donors and 8 of 10 granulocytapheresis donors experiencing reactions, but in none of 140 non-reacting controls. It is estimated that as many as %1.0 of all repeat apheresis donors may become sensitized to ethylene oxide and experience allergic symptoms during donation. Two of the 10 individuals experiencing reactions during granulocyte donations had positive skin tests to hydroxyethyl starch, and appeared to become sensitized to this red cell sedimenting agent during apheresis. Although none of these reactions have been life-threatening, the DTM is continuing to test apheresis donor for sensitization to ethylene oxide and hydroxyethyl starch, and to defer sensitized individuals from subseqent CS-3000 donations.